Pressure fluid motor



Dec.-17, l-EHJU. Q CURTls 2,224,858

PRESSURE FLUID MOTOR In were J'afe (Y 672572223.

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Dec. 171, 1940. J c, CURTIS 2,224,858

PRES SURE FLUID MOTOR Filed July 8, 1937 2 sheets-sheet 2 Patented Dec. 17, 1940 UNITED STATES PRESSURE FLUID MOTOR John Clark Curtis, Claremont, N. H.,-arssignor to Sullivan Machinery Company, acorporation of Massachusetts Application July 8', 1937, Serial No. 152,608 11 claims. (ci. lai-19)` This invention relates to pressure uid motors, and more particularly to improvements in pressure fluid Vmotors of 'the reciprocating-piston type Yespecially designed for use in hammer rock drills.

An object ofthis invention isvto provide an improved pressure fluid motor; Another object isto provide an improvedV` pressure fluid motor having improved fluid distribution means. Yet

ranother object is toprovide an improved pressure iluid motor of the pressure-iluid-actuated, reciprocating-piston' type especially designed for use in hammer rock drillskwhereby a relatively powerful impact blow and increased efficiency are attained.

A still further object it tov provide an improved pressure fluid motor having an improved fluid distributing valve and associated passage arrangement whereby, not only is the efliciency of the motor greatlyincreased', but also reduced fluid consumption may be attained. Other uses and advantages of the invention Will, however, hereinafter more fully appear.

In theV accompanying'drawings there is shown for purposes of illustration one form which' the inventionv may assume in practice.

In these drawingsf Fig. 1 is a view in longitudinal vertical section through ank illustrative embodiment of the imroved pressure'fluid motor. l

Fig. 2 is a view similar t0 Fig. 1, showingcer'- tain of the movingmotor parts in different positions. n

Fig. 3 isa rear end elevational view of the iluid distributing valve;

Fig.4 issa front end elevational view of the fluid' distributing valve.

Fig; 5 is'a cross sectional view taken' substantially on line 5--5 of lFig. 1.

Fig. 6- is a developed Alongitudinal sectional view taken substantially on line 6-6 of Fig.' 5.

' Fig. 7 is la side elevational view of the valve seat plate.

Fig` 8 is a sidel elevational View of the uid H distributing valve;

Fig..9 is. a frontend elevational view of the valve block cover.

Fig; `lO'is a rear end elevational Viewv ofthe valve-seat plate. Fig..l'l is anY enlarged fragmentary sectional Cal View oi ftlievalve, taken on the plane of Fig. l,

showing .the valve in its central position.

In this illustrative embodiment, there is shown a pressure fluid motor, generally designated I, herein of the' reciprocating-piston, hammer type,

.a dowel ypin 25. Engaging the collar 22 is a especially'designed for use in hammer rock drills, although it Will 'be evident that various features offthe invention may be incorporated in motors ofvarious other'types.

The improved pressure fluid motor, as shown 5 in the drawings, specifically comprises a ymotor cylinder-Z'having a bore 3 containing a reciprocatory motor piston 4. In this instance, the motor piston is of the hammer type' having a piston' head 5 litting the cylinder bore and a 10 forwardlyipro'jecting cylindric portion 6, in the -formjofastriking bar, adapted to deliver impact blows'to a Working tool'operated by the motor. The cylindric bar portion 6 of the piston is e.. guided in a bore 'I Aformedfin a front buffer ring 15 8, the latter also constituting the front cylinder head.` The cylinder is formed with a rearward extension 9 having a bore Ill alined with and of `somewhatlarger diameter than the cylinder bore 3; and arranged-in the bore I0 and abutting a shoulder I I at the Vjuncture of the bores, is a valve seat plate I2, the latter also constituting the rear cylinder head. The plateV I2 has formed on its rearward vface an annular shoulder I3and` in- Y wardly of said shoulder and projecting rear- 25 wardly from the body of the plate is an axial,

`sleeve-like portion I4 formedwith an external annular flange I5. Also arranged in the extension bore 'I0 and abuttingthe rear face of the plate I2 is avalve block Iiiv having bores I'I and I8 partially separated byan inwardly directed annular ange I9, the latter having a bore 20 of susbtantially smaller diameter than the bores I1 and I8. AAbutting the reariace of the valve block I e and also arranged in the extension bore I0 is ka valve block cover 2|. 'Arranged within the rearward portion of the extension bore IIJ is a ratchet collar 22, while interposed between this collar'and therear face ofV the cover 2| is a ratchet ring 23. The rvalve blockcover, ratchet ring and collar are maintained against rotative movement in the bore I 0 by a key pin 24 seated in registering grooves in the cooperating parts'. The cover 2| is fixed to the valve block I6 by means of back head 26 for holding the parts in assembled relation within the extension bore I0, and this fbackjhead and th'ebuiier ring 8 are clamped to the opposite-ends of the cylinder in any appropriate manner, as by usual side rods 21, 21 (see Fig. 5).

As is usual in`- motors adapted for use in hammer rock drills, means is provided associated with rthe motor piston and loperated thereby,l for 55 effecting rotary motion of the piston for the well known purpose of rotating the working tool. This tool rotation means comprises, in addition to the ratchet ring 23, a pawl carrier 29 carrying usual spring-pressed pawls engaging the teeth of the ratchet ring 23, and formed integral with this carrier is a riiie bar 36 having spiral grooves slidingly interlocked with the spiral keys of a rifle nut 3| fixed within the motor piston. The rifle bar at the opposite sides of the pawl carrier 29 is journaled within bores respectively formed in the sleeve-like portion I4 and in the ratchet collar 22, in the manner illustrated. As the motor piston moves forwardly toeffect its working stroke, the pawls of the pawl carrier slip'over the ratchet teeth so that the forward movement of the piston is unimpeded, and, upon reversal of movement of the motor piston, the pawls of the pawl carrier engage the ratchet teeth to holdthe rifle bar against rotation, and, as a result, due to the spiral groove and key interlocking connection of the rifle bar with the piston, the latter is rotated. The rotary motion of the motor piston isl transmitted to the working tool in a manner well understood by those skilled in the art. As -this tool rotation means as shown is of a conventional form, further description and illustration thereof are unnecessary.

Now referring further to the specic structure of the improved pressure fluid motor, and more particularly 'to the improved fluid distributing valve mechanism, it will be observed that reciprocably mounted in the valve chamber provided by the cooperation of the parts I2, I6 and 2I is a fluid distributing valve, generally designated 33, having an enlarged external flange 34 near its forward face fitting the front bore I8 of the valve block I6 and a sleeve-like body portion 35 fitting the bore 26 of the internal flange I9 of the valve block. Formed integral with the sleevelike body portion 35 of the valve is an inwardly directed annular flange 36 having its inner peripheral edge cooperating with the exterior periphery of the flange I5 of the plate body I4, for a purpose to be later explained. Formed in the opposite sides of the valves are internal bores 31 and 38, the latter being of slightly smaller diameter than the former, and being the rearward one of the two. The bore 3l of the valve has, when the valve is in the position shown in Fig. La sliding iit with the shoulder I3 on the plate I2, while the valve bore 38 at the opposite side of the valve flange 36 has, when the valve is in the position shown in Fig. 2, a sliding fit with the exterior peripheral surface 39 of an annular projection 40 formed integral with and projecting forwardly from the forward face of the cover`2I. The bores 31 and 38 within the valve at the opposite sides of the internal valve ange 36 are counterbored at 4I and 42 vnear the opposite end Valve faces respectively. The forward face of the external valve flange 34 is annularly recessed at 43 to obtain lightness. Formed in the valve block I6 are ports 44 for connecting the valve block bore I1 with ports 45 formed in the cylinder, and the latter communicate with passages 46 connected by ports 4l with the forward end of the cylinder bore, while an annular recess 48 formed in the rear face of the plate -I2 communicates with the front valve block bore I3 at the forward side of the valve, and this recess is connected by passages 49 with the rear end vof the cylinder bore. As shown most clearly in Fig. 6, communicating with the front valve block ,bore I6 at the rear side of the external valve flange 34 is a port 50 in turn communicating with a throwing passage 5I connected at its forward end by a port 52 with the cylinder bore somewhat nearer the rear than to the forward end of the latter. The throwing passage 5I is connected to atmosphere by a restricted leak passage 53. The cylinder bore 3 is annularly grooved at 54 substantially midway between the ends thereof, and this groove is connected to latmosphere through an exhaust port 55.

The means for supplying pressure fluid to the valve chamber comprises a rotary throttle valve 60 arranged in a bore 6 I formed in the back head 26, and this valve is formed with an internal pressure chamber 62 connectible by a passage 63 with a passage 64 communicating with a pressure chamber 65 formed within the ratchet collar and back head. This pressure chamber is connected by a series of passages 66 with an annular recess 6'I formed in the front face of the ratchet collar, and this recess communicates through the spaces between the ratchet teeth with an annular recess 68 formed in the rear face of the valve block cover 2|. The recess 68 is connected by a series of passages 69 with an annular recess 'I0 in turn connected through a large axial passage 'II extending through the annular projection of the cover, with the valve bore 38 at the rear side of the internal valve flange 36. Pressure may flow from the rear valve bore 38 through the restricted clearance space 12 between the inner peripheral edge of the valve flange 36 and the exterior periphery of the flange I 5, to the valve bore 3l at the forward side of the internal valve flange 36. The rear face of the plate I2 is preferably annularly recessed at 'I3 to increase the volume of the front internal pressure chamber. The distributing valve is guided within the valve chamber exteriorly solely at the outer periphery of the external valve flange 34 and the valve body 35, and the sliding fit which exists in certain relative positions between the uncountersunk portions of the walls surrounding the valve bores 3l and 38, and the exterior peripheries of the annular shoulders I3 and 39 is relatively free, but still an extremely close fluid-tight fit. The inner peripheries of the counterbores cooperate with the external peripheries of the annular shoulders I3 and 39 to provide for metering, so that the flow of pressure iiuid past the opposite ends of the valve is always the same regardless of any wear of thc valve. It will be noted that as the valve moves between its opposite positions, its walls just inwardly of the counterbores 4I and 42 will be momentarily simultaneously in contact with the exterior peripheries of the shoulders I3 and 39 respectively (see Fig. 11), and as a result, waste of Vpressure uid due to simultaneous admission to both ends of the cylinder, will be avoided.

The mode of operation of the illustrative form of the improved pressure fluid motor is as follows:

When the throttle valve 'B0 is in its on position shown in Fig. 1, pressure iiuid may iiow from the interior of the valve through the ports 63, 64, pressure chamber 65, passages 66, annular recess 6l, through the spaces between the ratchet teeth, annular recess 68, passages 69, recess 10, and the large axial passage 'II to the bore 38 within the rear end of the valve, and also through the somev what restricted annular space l2, to the bore 31 flange 36 through the metering space between the wall ofthe counterbore 42 and` the exterior; periphery ofthe annular'shoulder:4,.past the rear face of the valve portion 35;through the. valve blockl bore I'|-,' and through-the ports 44 and 45 and passages 45 and portsd'l, to the front end of the cylinder bore. At this time, the rear'end of the` cylinder bore will be `connected to the atmosphere through the exhaust groove 54 and exhaust port 55. Pressure fluid acting on the forward pressure area ofthe motor piston will move the latter rearwardly to effect its rearward stroke, and when the exhaust groove 54zis closed by the piston head, the fluid trapped in therear end of vthe cylinder bore will be compressed by the motor piston. The port 53 will bleed away any fluid which mayI tend to be compressed within the port 52, passage 5l, port 5G, and the space to the rear of the flange 34 on the valve. Continued rearwardV movement of the piston will cause the front edge of the pistonv head to uncover the forward edge of the exhaust groove 54, connecting the front end of the cylinder bore to exhaust, and the valve 33y will then immediately shift from the.l positionshown in Fig. 1 to thatshown in Fig. 2; this throwing of the valve being effected in part by the built-up compression pressure transmitted tothe forward face of the flange 34-through the ports 49 and annular groove 48, and in part by the reduction in pressure to the rear of thevalve occasioned by the connection of the'space within the cylinder bore at the yfront side of the piston with exhaust. Obviously, depending upon the proportions ofv the parts, throw wholly by compression pressure, or in part by compression pres'- sure and in part by reduction in pressure at the opposite (rearward) side ofthe valve, may be caused to take place. In any event, the valve will be shifted so promptly after uncovering of the groove 54 by. the rearwardly moving piston commences, that no waste'of pressure uid will occur.

Forward-movement of the motor, piston upon its blow-striking stroke will be effected by fluid admitted through the passage 1B, through the somewhat restricted annularopening 12, through the bore 31, .through the counterbore 4l, which provides a metering of the fluid for the forward working stroke of the piston, through the annular v space 48,- and the passages 45). A pressure differential exists in the'spaces to the front and rear of the ange 36, due to the flow through the restrictedpassage 12', which ,tends toI move the valve 33 to its forward position, but the valve is held in its rearward position by virtue of the larger pressure area. at theforward side of the valve. l

As the. motor piston `moves forward, its rear end will uncover the mouth of the 'port 52, and cylinder pressure will -passthrcugh the port 52, the passage 5l, and the passage- 5B, to act upon the rear side of the flange 34fon the valve 33, and will cause the valve 33 to be moved forward' to cut off the supply of motive fluid to therear end of the cylinder, and 'assume its position for the rearward stroke, desirably shortly before the rear end of the piston reaches the rearward edge of the exhaust groove 54. The mode of .operation of succeeding cycles will be understood from what has been so far said.

Ey virtue of the slight restriction of ow through the annular space 12, it will be observed that if the distributing valve should happen to be in the position of Fig.12, and the piston in the forward end of the cylinder with the rearend of the cylinder open to exhausawhen thethrottle valveris..opened,ithe valve 33, due to the'large differential in pressure: which. would exist between thelv spaces infront of. and to the rear of the flange 3S', will be promptly moved to, the position of Fig. 1, and set the' motor in operation. The vent port 535. also plays the additional function of preventing the building up of leakage pressures which might interfere withthe startingof the motor.

It willbe noted that in the preferred embodiment described .and illustrated, the distributing valve is. shifted, upon the'forward movement of the motorpiston, by pistoncontrolled pressure,

and upon the rearward pass of the motor piston, by acombination of the building up of a compression pressure and the relieving of an opposing pressure as the'exhaust port is reached and uncovered by the rearwardly moving piston.

Asa result of. this invention, it. will be noted that 1an improved. pressure fluid motor is provided,'having a novel `and improved fluid distribution means', `whereby certainty of starting is assured, whereby power may be increased, and whereby air consumptionlmay be maintained at a minimum. with respect to the power delivered. These and other uses and advantages of the improved pressure fluid motor will be clearly apparent to those skilled infthe art.

While `there is inthis application specifically described one form which the invention may assume in practice, it will be understood that this form of the same is shown for purposes of illustration and that the invention may bemodified and embodied in various other formsA without depended/ claims' What I claim as new and desire to secure by Letters Patent is:

1. In a pressure fluid motor, in combination, a cylinder having a bore7 a piston reciprocable in said bore, and iiuiddistribution means for supplying pressure fluidv to and exhausting fluid from said cylinder `bore for effecting reciprocation of'said piston 'comprising means providing avalve chamber having valve seat surfaces at its oppositeY ends, a Vfluid distributing Avalve movably mountedV in saidvalve chamber, 4and having seating surfaces at its'remoteextremities respectively adapted to seat against said valve seat surfaces, means cooperating with said valve providing a pair of internal pressurechambers separated by a restricted clearance space,l andl means forming uid supply passages connecting the opposite ends of said valve chamber respectively to the opposite lenols of rsaid cylinder bore, said valve controlling the flow of pressure fluid from said internal pressure chambers through the opposite ends of said valve chamber past said valve seating surfaces to said supply passages.

2. In .a pressure fluid.rnotor,.in combination, a cylinder having a bore, a piston reciprocable in said bore, and fluid `distribution means for supplying pressurefluidto' and exhausting fluid from said cylinder bore for effecting reciprocation of saidf piston comprising means providing a valve chamber having valve seat surfacesat its opposite ends, a fluid distributing valve movably mounted in said valve chamber and lhaving valve seating surfaces at itsv remote extremities respectively adaptedto seat against said valve seat surfaces, saidvalve having an annular flange pro' jecting inwardly therein and arranged between v parting from its spirit or'the scope of the ap- Y.

said valverseati'ng surfaces, means cooperating clearance space therebetween, the pressure fluid flowing from one pressure chamber to the other through said clearance space, and means forming fluid supply passages connecting the opposite ends of said valve chamber respectively to the opposite ends of the cylinder bore, said valve controlling the flow of pressure fluid from said internal pressure chambers through the opposite ends of the valve chamber past said valve seating surfaces to said supply passages.

3. In a .pressure fluid motor, in combination, a cylinder having a bore, a piston reciprocable in said bore, and fluid distribution means for supplying pressure fluid to and exhausting fluid from said cylinder bore for effecting reciprocation of said piston comprising means providing a valve chamber having valve seat surfaces at its opposite ends, said valve chamber having an enlarged bore and a reduced bore, a fluid distributing valve reciprocably mounted in said valve chamber and having an external flange slidingly fitting said enlarged bore and a sleeve-like body portion slidingly fitting said reduced bore, said sleeve-like body portion having valve seating surfaces at the remote extremities thereof respectively adapted to seat against said valve seat surfaces, and means providing fluid supply passages leading from the opposite ends of said valve chamber respectively to the opposite ends of said cylinder bore, said valve controlling the flow of pressure uid through the opposite ends of said valve chamber past said valve seating surfaces to said supply passages.

4. In a pressure fluid motor, in combination, a cylinder having a bore, a piston recprocable in said bore, and fluid distribution means for supplying pressure fluid to and exhausting fluid from said cylinder bore for effecting reciprocation of said piston comprising means providing a valve chamber having valve seat surfaces at its opposite ends, said valve chamber having enlarged end bores and a reduced intermediate bore, means providing fluid supply passages for connecting said enlarged bores respectively with the opposite ends of said cylinder bore, and a fluid distributing valve reciprocably mounted in said valve chamber and having an external flange slidingly fitting one enlarged bore and a sleeve-like body portion slidingly fitting said reduced bore and projecting within said other enlarged bore, said sleeve-like body portion having valve seating surfaces at its remote extremities respectively adapted to seat against said valve seat surfaces, said valve controlling the flow of pressure uid through the opposite ends of said valve chamber past said valve seating surfacesl to said supply passages.

5. In a pressure fluid motor, in combination, a cylinder having a bore, a piston reciprocable in said bore, and fluid distribution means for supplying pressure fluid to and exhausting fluid from said cylinder bore for effecting reciprocation of said piston comprising means providing a valve chamber having valve seat surfaces at its opposite ends, said valve chamber having enlarged and reduced bores, a fluid distributing valve reciprocably mounted in said valve chamber and having an external flange slidingly fitting the enlarged bore and a sleeve-like body portion slidingly fitting the reduced bore, said sleeve-like body portion having valve seating surfaces at its remote extremities respectively adapted to seat against said valve seat surfaces and said valve having an internal flange projecting inwardly therein and spaced inwardly of said valve seating surfaces,

means cooperating with said valve providing a pair of internal pressure chambers within the valve, said internal valve flange separating said chambers and providing a restricted clearance space therebetween, the pressure fluid flowing from one pressure chamber to the other through said clearance space, and means providing fluid supply passages connecting the opposite ends of the valve chamber respectively with the opposite ends of the cylinder bore, said valve controlling the flow of pressure fluid from said internal pressure chambers through the opposite ends of said valve chamber past said valve seating surfaces to said supply passages.

6. In a pressure fluid motor, in combination, a cylinder having a bore, a piston reciprocable in said bore, and fluid distribution means for supplying pressure fluid to and exhausting fluid from said cylinder bore for effecting reciprocation of said piston comprising means providing a valve chamber having valve seat surfaces at its opposite ends, a fluid distributing valve reciprocably mounted in said valve chamber and having valve seating surfaces at its remote extremities respectively adapted to seat against said valve seat surfaces, means cooperating with said valve inwardly of said valve seating surfaces providing metering spaces through which fluid must flow to the opposite ends of said valve chamber, and means providing fluid supply passages for connecting the opposite ends of said valve chamber respectively to the opposite ends of said cylinder bore, said valve controlling the ow of fluid through said metering spaces and past said valve seating surfaces through the opposite ends of said valve chamber to said supply passages.

7. In a, pressure fluid motor, in combination, a cylinder having a bore, a piston reciprocable in said bore, and fluid distribution means for supplying pressure fluid to and exhausting fluid from said cylinder bore for effecting reciprocation of said piston comprising means providing a valve chamber having valve seat surfaces at its opposite ends, a fluid distributing valve reciprocably mounted in said valve chamber and slidingly guided in lsaid chamber solely at its exterior periphery, said valve having valve seating surfaces at its remote extremities respectively adapted to seat against said valve seat surfaces and provided with internal. bores within its opposite ends inwardly of said valve seating surfaces, means at the opposite ends of the valve chamber providing a sliding fit with said valve bores respectively, said valve bores, as the valve reciprocates, moving alternately into sliding engagement with said sliding-t-providing means, and means forming fluid supply passages connecting the opposite ends of said valve chamber respectively to the opposite ends of said cylinder bore, said supply passages having fluid flow to them controlled by said valve and fluid flowing through said supply passages flowing past said sliding-flt-providing means and said valve seating surfaces in the different seated positions of said valve.

8. In a pressure fluid motor, in combination, a cylinder having a bore, a piston reciprocable in said bore, and fluid distribution means for supplying pressure fluid to and exhausting fluid from said cylinder bore for effecting reciprocation of said piston comprising means providing a valve chamber having valve seat surfaces at its opposite ends, a fluid distributing valve reciprocably mounted in said valve chamber and slidingly guided in said chamber solely at its exterior periphery, said valve having valve seating surfaces at its remote extremities respectively adapted to seat against said valve seat surfaces and provided with internal bores within its opposite ends inwardly of said valve seating surfaces, means at the opposite ends of the valve chamber providing a sliding iit with said valve bores respectively, said valve bores, as the valve reciprocates, moving alternately into sliding engagement with said sliding-fit-providing means, means forming fluid supply passages connecting the opposite ends of said valve chamber respectively to the opposite ends of said cylinder bore, said supply passages having fluid flow to them controlled by said valve and fluid flowing through said supply passages flowing past said sliding-t-providing means and said valve seating surfaces in the different seated positions of said valve, and means cooperating with said sliding-fit-providing means for metering the fluid flowing past said valve seating surfaces at the opposite ends of said valve.

9. In a pressure fluid motor, in combination, a cylinder having a bore, a piston reciprocable in said bore, and fluid distribution means for supplying pressure fluid to and exhausting fluid from said cylinder bore for effecting reciprocation of said piston comprising means providing a valve chamber having valve seat surfaces at its opposite ends, a uid distributing valve reciprocably mounted in said valve chamber and slidingly guided therein solely at its exterior periphery, said valve having end seating surfaces at its remote extremities and provided with internal bores within its opposite ends inwardly of said end seating surfaces, means at the opposite ends of the valve chamber providing a sliding fit with said Valve bores, said valve bores, as the valve reciprocates, moving alternately into sliding engagement with said sliding-iit-providing means, means cooperating with said valve bores and said sliding-flt-providing means providing an internal pressure fluid supply within the valve, and means forming fluid supply passages connecting the opposite ends of said valve chamber respectively with the opposite ends of said cylinder bore, said valve controlling the flow of pressure fluid from said internal pressure fluid supply past said sliding-fit-providing means and the seating surfaces at the opposite ends of the valve to said supply passages. f

10. In combination, in a pressure uid motor, a cylinder, a piston therein, and fluid distribution means for said motor including passage means for supplying operating fluid respectively to the opposite ends of said piston, an operating fluid supply, and valve means for controlling the communication of said passage means with said fluid supply, said valve means having end seating surfaces at the remote extremities thereof, and means cooperating with said valve means inwardly of said end seating surfaces thereof providing metered ow of operating fluid to the opposite ends of said cylinder, said valve means so formed and so arranged with respect to said cooperating metering-providing means as to preclude simultaneous communication of both of said passage means with said fluid supply.

11. In a pressure fluid motor, incombination, a cylinder having a bore, a piston reciprocable in said bore, and iiuid distribution means for supplying pressure fluid to and exhausting fluid from said cylinder bore for effecting reciprocation of said piston comprising means providing a valve chamber having valve seat surfaces at its opposite ends, a fluid distributing valve reciprocably mounted in said valve chamber and having valve seating surfaces at its remote extremities respectively adapted to seat against said valve seat surfaces, means cooperating with the opposite ends of said valve inwardly of said valve seating surfaces providing metering spaces through which iuid must flow to the opposite ends of said valve chamber, and means providing fluid supply passages for conducting pressure fluid from the opposite ends of said valve chamber respectively to the opposite ends of said cylinder bore, said valve controlling the flow of pressure fluid through said meteringfspaces past said valve seating surfaces to the opposite ends of said valve chamber, and said metering spaces and the opposite ends of the valve being so spaced that when the valve is in its central position during movement of the valve between its opposite seated positions, flow of pressure fluid through both metering spaces is momentarily completely cut off.

JOHN CLARK CURTIS. 

